Our recent findings made during this funding period have revealed an unexpected dependence of canonical Burkitt's Lymphomas (BLs) and Wp-restricted BLs on EBV's BART miRNAs (Vereide and Sugden, 2011; Vereide et al., submitted). We have also found that EBV's miRNAs promote transformation of newly infected B-cells (Seto et al., 2010; Vereide et al., submitted). We propose to identify the mRNAs that are regulated by EBV's miRNAs to drive tumor maintenance and promote transformation. EBV encodes 25 pre-miRNAs which can yield 50 mature miRNAs. These miRNAs have predicted seed sites in more than 30% of all human mRNAs. However, many studies, including our own, indicate that the vast majority of these mRNAs are not regulated by EBV's miRNAs (Kuzembayeva et al. submitted; Dolken et al. 2010). It is therefore essential to develop functional assays to allow characterization of presumptive targets of miRNAs in order to learn if the subtle differences in their expression mediated by miRNAs have functional consequences. We have developed functional assays for tumor maintenance and transformation dependent on EBV's miRNAs when the miRNAs are expressed at physiological levels. This latter qualification is important because miRNAs act in a dose-dependent manner, and EBV's miRNAs are often expressed at low levels (Pratt et al., 2009). We propose to identify minimal subsets of EBV's miRNAs that regulate a given phenotype and then develop matched pairs of cells that do and do not express these miRNAs from EBV plasmids. The mRNAs in the RISCs immunoprecipitated from these pairs of cells will be identified and enumerated by deep sequencing, sorted bioinformatically, and their 3'UTRs tested in reporter assays for regulation by specific viral miRNAs. We have used all of these assays successfully in our identification of caspase 3 as a target for BART miRNAs in canonical BLs (Vereide et al. submitted). The mRNAs found by this set of assays will then be tested functionally for potential contributions to maintaining lymphomas and transforming primary B-cells.